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Water cooling multiplies brake resistor power density

13 June 2016

There would seem to be a world of difference between the current generation of advanced electric sports cars and the heavy industries where Leicester based electrical firm Cressall Resistors traditionally operates.

However, the common denominator is a lightweight 25kW water-cooled resistor, as Martin Nicholls of Cressall explains. 

In the middle of the last decade electric sports, with acceleration comparable to a normal internal combustion engine sports car, began production and there are now a number of very advanced models on the market. They do of course, come with the need for a similarly sophisticated wallet.  

Cressall Resistors’ involvement in the electric vehicle market allowed it to identify the need for the first of a series of Electric Vehicle (EV) resistors, culminating in a product the company christened the EV2. Electric cars have always been at a disadvantage against their petrol-driven counterparts in terms of energy density. A can of petrol weighing 4kg equates to an electric vehicle battery weighing 500kg and costs six Euros compared to 40,000. 

This does not take into account the cost of charging, which is not too dissimilar to the cost of petrol. This is because when you buy electricity you are effectively paying three times as much as for the raw material due to the inefficiency of power generation. 

The issue of battery life exacerbates high capital cost and weight. As a result, many all-electric sports cars, such as the Tesla, are status symbols more than economic ways of getting to work. Therefore, the whole idea of a hybrid car is that it runs from a much smaller engine of say 10kW, which is appropriate for the power consumed in an average car journey and which recharges the battery. 

Advanced ceramic

In the meantime, Cressall has identified an industrial need for water-cooled resistors with minimal footprints in industrial applications, launching the EV2 water-cooled resistor suitable for medium voltage industrial applications. This features two plates made from an advanced ceramic with innovative properties. 

An important benefit of using an industrial electric drive is that reliable systems of regenerative and dynamic braking are available to complement or replace traditional mechanical braking systems. The advantages of electric braking include control, reliability, mechanical simplicity, weight saving and in certain cases, the opportunity to make use of the regenerated braking energy to top up batteries.

On the one hand, the EV2 has the same insulation properties as any other ceramic, with insulation withstand to 20kV. However, the thermal properties are such that it will conduct heat almost as effectively as aluminium - normally a good insulator is a bad conductor. 

Originally developed for the electronics on the Japanese Shinkansen bullet train, it allows power electronic components to be mounted on one side and a copper heat sink on the other, so it is not only an electrical insulator but also a thermal conductor. 

The evolution of EV2 resistors
The evolution of EV2 resistors

The EV2 was developed using novel patented construction to manage temperature. This design was prepared using finite element analysis (FEA) software tools, allowing a peak temperature of about 350°C from an electrical load of over 25kW, although 40kW has been attained. 

Launch customers for the EV2 include Rolls Royce in Canada and Samsung for a Korean vehicle prototype. This is driven by the need for minimising onboard space previously occupied by traditional water-cooled resistor designs. Because the resistor is completely encapsulated, it has a lower explosion risk for offshore rigs and can conduct heat away more quickly from areas where this would be a problem, like the bowels of a ship for example. 

The search for a material with higher resistivity than stainless steel led Cressall to consider the use of graphite such as that traditionally used in vacuum furnace elements. Graphite is very stable and has desirable ohmic values but being pure carbon, it oxidises. Cressall has now found a way of encapsulating a graphite element within EV2 to protect it from the air. 

Cressall’s French distributor has a customer that makes winches for offshore oil platforms, which use Caterpillar drives to lower pipes down to the seabed, and this requires braking. The customer now uses Cressall water-cooled resistors at the same price as the traditional air-cooled resistors but occupying one tenth of the space and the company has plans to get certifications from both Lloyds and Atex for the EV2. 

Independent braking systems

In 15-20 years, we will probably all be driving hybrid or electric vehicles and all vehicles must have at least two or three independent braking systems for safety reasons. Typically, in a conventional car these are dual-circuit hydraulic brakes plus a handbrake.

Wherever possible, braking will be regenerative rather than mechanical: this creates the possibility of storing and re-using the regenerated braking energy, rather than just dissipating it as waste heat. Storage of the recovered energy can be in the vehicle’s batteries or in ancillary media such as flywheels or ultracapacitors.

Cressall’s EV2 water-cooled resistor is a 25kW unit available as a single unit or as a block of ten with a common cable box attached to 250kW braking power input, all using a common water supply. Cooling is achieved on the EV2 by pumping cold water, which comes into one end of the system and then absorbs the heat. It can be pumped through a radiator, which can be located some way from the heat generating equipment. 

This hot water can then be used to provide heat to the cabin of the vehicle. This innovative method of heating reduces the amount of energy required from the battery and uses heat that otherwise would have been wasted.  

Perhaps the needs of the automation industry and the electric vehicle sector are not worlds apart after all?

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